Electrically combining two phase shifted or time delayed signals, such as two stereo audio signals, for example, can create significant unwanted distortions and artifacts. A distinguishable loss or change in the high frequencies, a "whirling" effect, or even a hollowness in the medium frequencies are examples of the distortion resulting from combining the signals. Frequency cancellations occur across the spectrum (the comb filter effect), and the frequencies at which the cancellations occur become lower and more noticeable as the time delay error or phase shift increases. The effect of the frequency cancellations and other degradations is a reduction in the quality and content of the information conveyed by the two signals. In the case of a monaural audio signal formed by combining the two signals, the effect is a significantly degraded sound quality. When phase shifted stereo audio signals are combined acoustically in space, the result of the combination is a rotation of the perceived acoustical image.
The sources of relative time delay errors between separate signals or channels are numerous. Much radio broadcasting is accomplished by use of stereo tape cartridges wherein each signal is recorded on a separate magnetic track. The heads which record the tracks or which transduce signals from the tracks may be out of perpendicular alignment with the longitudinal extent of the tape (an "azimuth" error) thereby introducing time delays and phase errors. Despite the best efforts at aligning the heads, it is virtually impossible to avoid the time delay errors created by azimuth errors, due to azimuth errors in the equipment which recorded the tracks, improper insertion of the recording medium into the playback machine, and still other azimuth errors in the playback machine, for example. Differences in propagation time between the two signals also create a time delay error. Propagation time differences result as the signal is transmitted through electronic equipment which has separate signal paths for each signal. Television signals, which also include stereo audio signals, that are reflected from satellites are particularly prone to time delay errors. Stereo signals which are simultaneously transmitted as discrete left and right channels over different microwave links are subject to inter-channel time delay errors due to different propagation path lengths or different transmission circuit characteristics. Other sources of time delay errors are in digital compact disc equipment, hi-fi VCRs, and multichannel cinema audio signals. Because most radio and television receivers, home video cassette machines and movie theaters are monaural, substantial signal quality degradation occurs due to the monaural summing of the two phase shifted signals.
While as a conceptual matter, correction of time delay and phase errors would appear desirable, certain counterbalancing considerations also exist. Normal stereo programming inherently incorporates normal phase fluctuations. These intentional phase fluctuations create the desired psychoacoustic effect. The phase differences or intentional time delay errors create two separate neurological inner ear sensations which have phase differences resulting from the slightly different audio path lengths (equivalent to auditory paralax). These sensations are recognized by the human brain as primary sound localization cues.
Spacial effects from the sound sources can only be humanly realized if there is some degree of correlation between the two separate signals. If the degree of correlation is increased to unity so that both channels are identical, the effect of a monaural sound source is created. Conversely, as the degree of correlation between the two channels or signals is reduced to zero, the impression of a psychoacoustic stereo image moves from a centralized location to a spacially distributed image and is ultimately replaced by the impression of two separate sounds from two separate sources. It is therefore important to maintain the intended correlated phase fluctuations inherent in stereo signals and broadcasts, but to eliminate the unintended relative time delay errors between the two separate signals to avoid degradation in the information those signals contained.
Attempts at solving the problem of unintended time delay errors have involved recording alignment pilot signals on each of the parallel tape tracks and attempting to use those alignment signals to correct for time delay errors. The disadvantage in this approach is that special equipment is required to decode the pilot signals, thereby limiting the usefulness of such a technique to those applications where the equipment is available. Another approach has been matrixing, where the sum of two signals is recorded on one track and the difference of the two signals is recorded in the other track. Matrixing simply re-introduces the same problem in another form, since time delay errors will lead to instability or degradation when decoding the matrixed information.
The most satisfactory solution to the time delay problem is that of using the program content or information contained in the two signals themselves. When properly extracted and processed, such information can be used to correct unintentional time delay errors. In pursuing a solution in this manner, however, it was recognized that it was necessary to preserve the normal phase fluctuations inherent in the correlated stereo signals. Thus, to correct for unintentional time delay errors in the two signals while preserving the normal phase fluctions, it was necessary to attempt to distinguish between these two types of time delay or phase error situations.
Some of the earlier time delay error correction systems, such as the Model 2000 and Model 2100 "Phase Chaser" manufactured by Howe Audio Productions of Boulder, Colo. 80301, both of which are prior art to the present invention, employed circuitry such as that shown in FIG. 1. A left signal and a right signal were respectively applied to the input terminals of amplifiers 12 and 14. The signals delivered from the amplifiers 12 and 14 were conducted through signal propagation control means, in the form of left and right voltage controlled time delay circuits 16 and 18, for controlling the relative time delay of the two signals which are fed to output driver amplifiers 20 and 22, respectively. The controllable time delay circuits 16 and 18 vary or control the delay or propagation time of each signal therethrough to the driver amplifiers 20 and 22, respectively. A feedback correction circuit, which comprises a cross-correlator 24 and a pair of integrators 26 and 28, delivers an error correction signal at 30 which is related to the average (i.e., integrated) relative phase difference to the two signals applied to the driver amplifiers 20 and 22. An inverting amplifier 32 alters the polarity of the correction signal at 30 to cause the left time delay circuit 16 to react oppositely in time delay or signal propagation effect compared to the right time delay circuit 18. Only one time delay circuit 16 or 18 operates at a time. The "leading" signal is delayed an amount to place it in a zero time delay relationship with the "lagging" signal. The two signals are thus brought into a non-phase shifted relationship.
More information concerning the typical prior art time base correction system shown in FIG. 1 can be found in the user's manuals published for the Models 2000 and 2100 Phase Chasers, and in a paper titled "Stereo Phase Error Detection and Automatic Phase Correction Using an Audio Cross-Correlation Technique" by David A. Howe, published in the Proceedings of the 39th Annual Broadcast Engineering Conference of the National Association of Broadcasters, Apr. 13 to 17, 1985, at pages 543 to 559.
The voltage controlled time delay circuits 16, 18 are well known in the art and are known as all pass filters. A description of one such voltage controlled time delay circuit can be found in the article by Mr. Howe referenced above. Other configurations suitable for this function include sampled input charged coupled device (CMOS bucket brigade) and standard audio digital delay line.
The cross-correlator 24 is a device whose output phase error signal at 34 is a function of the relative phase difference or error between its two input signals, i.e. those supplied at the input to the driver amplifiers 20 and 22. The phase error signal at 34 from the cross-correlator 24 represents the degree of coherence (or similarity between both real and imaginary components) existing between the two input signals. An explanation of the operation of a cross-correlator 24 can be found in the paper by Mr. Howe referenced above, and in an article "Frequency Shifter Encompasses Audio Band" by Franklin G. Fink, published in EDN, Oct. 27, 1982, at page 244. An explanation of the use of Wein bridge networks as 90 degree phase splitting networks, required for analog cross-correlation, can be found in "Wide Band Phase Shift Networks" by R. B. Dome, published in Electronics, December 1946, beginning at page 112 and in "Properties of Some Wide-Band Phase Splitting Networks: by David G. C. Luck, published in Proceedings of the I.R.E., February 1949, beginning at page 147. Details of 90 degree phase splitting networks employing cascaded single pole all-pass filters can be found in "Normalized design of 90 Degree Phase Shift Networks" by S. D. Bedrosian, published in IRE Transactions on Circuit Theory, June 1960, beginning at page 128, and in "Cascaded active circuits yield 90-degree phase difference networks" by Raymond E. Cook, published in EDN, April 1973, beginning at page 52. Proper operation of the cross-correlator can be obtained by examining the frequencies in the two signals in the rand of 900 Hz to 20 kHz.
The concept behind the two integrators 26 and 28 was to attempt to correct for the unintentional relative time delay error between the two signals, while recognizing that it was desirable to preserve the intentional normal phase fluctuations inherent in the correlated stereo signals. It was presumed that the unintentional time delay behaves more like a fixed relative time delay (created by a fixed azimuth error, for example) than a relatively fast or rapidly changing time delay. Accordingly, the slow integrator 28 derived a correction signal by slowly integrating the phase error signal at 34 to correct for the unintentional generally fixed phase delay. The effect of the slowly derived correction signal from the integrator 28 was to change the relative time delay between the left and right signals by a considerable amount, up to 100 microseconds, for example. The intent of the fast integrator 26 was to adjust for relatively rapidly varying phase fluctuations such as those which occur from tape flutter and "wow" associated with tape playback machines. Unfortunately, the rapid phase fluctuations inherent in tape wow and flutter are very similar in nature to the normal phase fluctuations in correlated stereo signals. Consequently the fast integrator also had the undesirable effect of degrading or eliminating the inherent desirable phase fluctuations, thus reducing the quality of the stereo image. The degradation was continuous because the correction signals from both integrators were added together and thus both correction signals had a continuous effect.
Prior art time delay error correction systems also lacked the capability to rapidly respond to abrupt changes in the magnitude or polarity of the unintentional time delays which may have been created by a variety of sources, for example splicing new material into pre-existing material. Ultimately, the slow integrator 28 would correct for such phase changes, but only after a significant time delay of, for example, up to 15 seconds. To adjust the integration rate of the slow integrator 28 so that its correction signal was more rapidly effective also had the undesirable effect of degrading the normal phase fluctuations inherent in the correlated stereo signals. Thus to increase the integration rate and achieve faster time base correction inherently involved the degradation or reduction of the psychoacoustic stereo image. The fast integrator 26 in such prior art systems, was simply incapable of accommodating the significant major phase changes, but was only effective in achieving correction for very minor phase fluctuations. The fact that the output of the fast and the slow integrators 26 and 28 was summed together to create single correction signal at 30 resulted in the major component of the correction signal being established by the slow integrator 28.
The prior art circuitry known to have been employed in the fast and slow integrators 26 and 28 is illustrated in FIG. 2. The phase error signal at 34 from the cross-correlator 24 is applied through resistors 40 and 42 to capacitors 44. The capacitors 44 are oppositely connected polarized electrolytic capacitors, but could be any non-polar capacitor. The positive input terminal of a voltage follower operational amplifier (op amp) 46 is connected to the junction of the resistor 42 and capacitors 44. The resistors 40 and 42, the capacitor 44 and the op amp 46 function as the slow integrator to gradually integrate the phase error signal at 34 from the cross-correlator 24 and supply an input signal to a driver op amp 48. The output signal from the op amp 48 is the control correction signal at 30. A capacitor 50, connected in parallel with a resistor 52 at the output terminal of op amp 46, allows the signal derived by the slow integrator to propagate slightly faster to the op amp 48. The function of the fast integrator is achieved by a feed forward resistor 54 which allows a small fraction of the unintegrated phase error signal 34 to reach the driver op amp 48. Directly feeding forward of a small fraction of the error signal allowed high frequency phase errors to be quickly corrected, although the resistor 54 did not, strictly speaking, achieve an integration function. In addition to compensating for the high frequency phase errors from tape wow and flutter, the feed forward resistor 54 also corrected at least a portion of the normal phase fluctuations inherent in the high frequency stereo signals, thereby degrading part of the information contained in the two signals and degrading stereo performance.